The present invention relates to apparatus for measuring fluid flow and, in particular, to an apparatus for the measurement of respiratory gases that are inhaled and exhaled from a human or other animal. Pneumotach apparatus of the type described herein include a pheumotachometer which is the flow measuring head and pneumotachographs which comprise equipment used in conjunction with the flow heads to provide a visual or recorded flow measurement to an operator.
Various types of pneumotach apparatus have been used for a number of years, especially in the measurement of respiratory gases so as to provide scientists, doctors, exercise physiologists and the like with a relatively accurate flow measurement of a person or animal undergoing various pulmonary physiological tests including physical exercise or stress inducing activities to determine the effect of such activities on the quantity of air or the like breathed by the person or animal. Such apparatus is also used in measuring resuscitator flows to patients during surgery, comas or related medical events where mechanically assisted breathing is required. For some tests, the exhaled gases are also analyzed to determine the component parts thereof and to allow a physician, scientist or other operator to better determine the metabolism and breathing capabilities of the party being tested.
For certain types of tests the need for improved accuracy in gas flow measurement is steadily increasing. Whereas at one time only a general or relative flow rate was required from devices designed and sized for a typical adult range of individuals, now tests have been devised to broaden that range to include, for example, children and animals and the accuracy of the flow measurements required has substantially increased.
A conventional pneumotach apparatus is shown in U.S. Pat. No. 3,626,755 of H. Rudolph which is incorporated herein by reference. Although the apparatus illustrated in the original Rudolph patent was quite satisfactory for the requirements of the industry for that period of time, as mentioned above, modern requirements are for a substantially improved measurement capability.
In particular, in the original device O-rings were usually utilized at the base of pressure sensing taps to seal these taps against annular rings supporting screens used therein. Unfortunately, because the annular rings have a curved surface and the taps have a planar surface nearest the annular rings, it was quite difficult for the O-rings to seat correctly and there was usually some leakage thereabout. Even a fairly minor leakage of this type can effect the measurement capability.
Secondly, in the original pneumotachs, it was possible for gas flows to flow between and around the outer annular surface of the screen support rings so as to by-pass the screens. Depending upon where this by-pass occurs, the resultant measurement varies and is inaccurate.
Thirdly, in the original pneumotach devices the plate for supporting the taps was independent from the remaining part of the outer housing for the device. This allowed leakage between the innerconnection of the parts.
Finally, it has been found that the tubular delivery of gases to and from the screens in an uncontrolled manner produces turbulence and other problems that effect flow measurement. In particular, the relatively flat ends of the tubes aligned parallel to the screens and the substantial spacing of these ends from the screens have been found to create turbulent flow and other problems in the vicinity of the screens, thereby effecting flow measurement.